Ii6 THE BIOLOGY OF TWINS 



SEX-DIFFERENTIATION IN MAMMALS 



Although sex is zygotically determined in mammals, 

 the differentiation of sex-characters depends on a 

 secondary mechanism that is believed to be associated 

 with an internal secretion of the gonads. It has been 

 long known that castration or ovariotomy of young 

 mammals prevents the development of adult, sexual 

 characters and the individual remains a neuter, though 

 zygotically either a male or a female. Steinach in a 

 brilliant series of experiments with rats has shown that a 

 transplantation of ovaries into a young castrated male 

 markedly alters the sex-differentiation of the operated 

 individual, making it take on many of the characters 

 of the female; even milk glands become functional 

 and a maternal instinct develops. Conversely, a transfer 

 of the testicular tissue into an ovariotomized female 

 tends to masculinize the animal, so that it becomes large 

 like the male and exhibits the pugnacious character 

 of the latter. Since only the glandular portion of the 

 transplanted ovary or testis survives in these experi- 

 ments there is no alternative but to attribute the reversal 

 of sex- tendency to some secretion of these glands, and, 

 for a lack of a better term, the active principle has been 

 called hormone. The interstitial glandular tissue of the 

 testis secretes, therefore, male-differentiating hormones 

 and the equivalent tissue of the ovary secretes female- 

 differentiating hormones. These hormones must be 

 given off into the blood, for it affects all parts of the 

 body. 



Crucial as are the experiments in transplantation of 

 gonads, they do not equal in subtlety and finish the 

 experiment of Nature performed in the case of the free- 



